Three-piece, wound golf balls with balata covers are preferred by many expert golfers. These balls provide a combination of distance, high spin rate, and control that is not available with other types of golf balls. However, balata is easily damaged in normal play, and, thus, lacks the durability required by the average golfer.
In contrast, amateur golfers typically prefer a solid, two-piece ball with an ionomer cover, which provides a combination of distance and durability. Because of the hard ionomer cover, these balls are almost impossible to cut, but also have a very hard "feel", which many golfers find unacceptable, and a lower spin rate, making these balls more difficult to draw or fade. The differences in the spin rate can be attributed to the differences in the composition and construction of both the cover and the core.
Many attempts have been made to produce a golf ball with the control and feel of a wound balata ball and the durability of a solid, two-piece ball, but none have succeeded totally. In various attempts to produce an ideal golf ball, the golfing industry has blended hard ionomer resins (i.e., those ionomer resins having a hardness of about 60 to 66 on the Shore D scale, as measured in accordance with ASTM method D-2240) with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been unsatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
While different blend combinations of species of one variety of polymer, such as prior art ionomers, i.e., copolymers of an olefin and an .alpha.,.beta.-unsaturated carboxylic acid, have been successfully used in the prior art, different polymers, such as carboxylic acid based ionomers and balata or other non-ionic polymers have not been successfully blended for use in golf ball covers. In general, prior art blends of polymer components are immiscible, i.e., heterogeneous on a microscopic scale, and incompatible, i.e., heterogeneous on a macroscopic scale, unless strong interactions are present between the polymer components in the mixture, such as those observed between carboxylic acid based ionomers and other polymers containing carboxylic acid groups. In particular, this lack of compatibility exists when an ionomer is blended with a polyolefin homopolymer, copolymer, or terpolymer that does not contain ionic, acidic, basic, or other polar pendant groups, and is not produced with a metallocene catalyst. These mixtures often have poor tensile strength, impact strength, and the like. Hence, the golf balls produced from these incompatible mixtures will have inferior golf ball properties such as poor durability, cut resistance, and so on. In contrast, a compatible blend may be heterogeneous on a microscopic scale, but is homogeneous on a macroscopic scale, and, thus, has useful golf ball properties.
In this regard, U.S. Pat. No. 5,397,840 discloses golf ball covers including a blend of "ionic copolymers" and "non-ionic copolymers". However, the "ionic copolymers" are defined as copolymers of an .alpha.-olefin and a metal salt of an .alpha..beta.-unsaturated carboxylic acid, and the "non-ionic copolymers" are copolymers or terpolymers containing ethylene or propylene and acrylic or methacrylic acid monomers. Therefore, strong interactions exist between the metal salts of the "ionic copolymers" and the acrylic or methacrylic acid monomers of the "non-ionic copolymers" that allow compatible blends to be formed. These interactions do not exist in prior art blends of ionomers and polymers that are truly non-ionic or nonpolar.
U.S. Pat. No. 5,616,640 to Harris et al. discloses golf ball cover compositions comprising an oxa acid compound having the formula ##STR1##
which may be blended with prior art, carboxylic acid based ionomers to provide golf balls having an excellent spin rate and good shear resistance.
Co-pending application Ser. No. 08/978,510 now U.S. Pat. No. 5,869,578 discloses golf balls comprising "saponified ionomers", i.e., ester based ionomeric polymers produced by carrying out a hydrolysis or saponification on copolymers containing pendant ester groups to form an ionomeric polymer that is less hydrophilic than typical carboxylic acid based ionomers to provide golf balls having enhanced physical properties when compared to prior art golf balls.
However, there is no known disclosure of golf balls comprising compatible blends of oxa acids and saponified ionomers.
Hydrolysis or saponification of alkyl acrylate units in a crosslinkable polymer chain is disclosed by Gross in U.S. Pat. No. 3,926,891. This is accomplished by dissolving the polymer in an aqueous alkali metal hydroxide solution and then heating. The product is recovered by coating the solution onto a substrate and evaporating the water or by extruding the solution into a non-solvent. In U.S. Pat. No. 3,970,626, Hurst discloses heating a mixture of an alkali metal hydroxide, a thermoplastic ethylene-alkyl acrylate copolymer and water to saponify the acrylate units and form an aqueous emulsion. This emulsion can be used as such, partially dried to a paste or moist solid, or fully dried to solid form.
A different approach to hydrolysis or saponification of an ethylene-alkyl acrylate copolymer is disclosed by Kurkov in U.S. Pat. No. 5,218,057, in which the copolymer is mixed with an aqueous solution of an inorganic alkali metal base at a temperature sufficient for saponification to take place and at which the copolymer undergoes a phase change. Typically, the copolymer would be molten when mixed with the aqueous solution.
Each of these prior methods, with the exception of that disclosed in co-pending application Ser. No. 08/978,510 now U.S. Pat No. 5,869,578, requires that the polymer component be in contact with water, either by conducting the reaction in an aqueous medium or by adding an aqueous solution to the polymer. Processes of this nature pose several disadvantages, however. First, it is difficult to remove water from the hydrolyzed or saponified polymer product. The polymer product is in the form of a salt that has a more polar nature than the reactant acrylate ester, and so is more likely to associate with or hydrogen bond to a polar solvent like water. The energy required to remove a highly interacting polar solvent like water is much greater than for a nonpolar or weakly polar organic solvent. Second, it is important to remove water from the ionomer product because the presence of water can have detrimental effects on ionomer mechanical properties imparted by the polar ionic domains, which act as the effective crosslink sites. Residual water weakens the ionic interactions within these domains, thereby reducing the mechanical property benefits the domains impart. Finally, incomplete removal of water can lead to difficulty in later fabricating steps where the product ionomer is reheated and shaped, e.g., into golf ball covers. Residual water can cause undesirable irregularities and imperfections on the surface of fabricated articles by the formation of blisters. Residual water within fabricated polymer articles can lead to void formation and even uncontrolled foaming with a concomitant undesirable influence on the mechanical properties, load bearing capacity and durability of the fabricated articles.
Melt state neutralization of an ethylene-acrylic acid copolymer by a solid, solution or slurry of an alkali metal salt is disclosed by Walter in U.S. Pat. No. 3,472,825. In the examples provided, hydrolysis is accomplished by mixing an alkali hydroxide with copolymer at constant temperature either in a Banbury mixer or on a two roll mill. Walter does not disclose the use of extrusion type polymer processing apparatus for this neutralization.
McClain, in U.S. Pat. No. 4,638,034, discloses a process whereby ethylene-acrylic acid copolymers or their ionomers are prepared from ethylene-alkyl acrylate copolymers by saponifying the latter in the melt with metal hydroxides to form an ionomer and a by-product, i.e., alkanol, then optionally acidifying the ionomer to form the free acid copolymer. This process proceeds in the molten state and in the absence of solvent or water, other than the by-product alkanol. Saponification proceeds under non-static mixing conditions, typically with equipment commonly employed in the art of mixing molten polymer materials such as multiroll mills, a Banbury mixer or a twin screw extruder.
The process disclosed by the '034 reference is, however, incapable of providing optimal product quality since blending and saponifying in a single operation as taught by the subject reference leads to rapid hydrolysis, with a concurrent rapid increase in viscosity. Due to this rapid increase in viscosity, the resultant mixture is non-uniform and therefore the physical properties of products made from this material are not consistent throughout the product.
During the melt state conversion of the alkyl-acrylate copolymer to the metal acrylate copolymer salt, a great decrease in melt flow rate occurs with a corresponding great increase in melt viscosity. While not wishing to be bound by any particular theory, this decreased melt flow rate is thought to occur because of the tendency of the relatively polar ionic salt functionalities formed during the saponification reaction to associate with themselves rather than the relatively nonpolar unreacted alkyl acrylate or comonomer chain segments. Aggregations of salt moieties arising from side groups attached to different chains into ionic domains introduces effective crosslink points throughout the molten copolymer. The effective crosslinks, in turn, greatly increase the copolymer melt viscosity and, correspondingly, greatly decrease copolymer melt flow rate.
A need exists in the golf ball art for highly durable golf balls, which have improved performance, and may be tailored to have virtually any combination of feel and spin rate. The present invention provides such a golf ball.